This invention relates generally to the field of fluid particle contact and more specifically to apparatus for the moving bed design of radial or horizontal flow fluid solid contacting devices. More specifically, this invention is related to an apparatus for the contacting of a hot fluid stream with particulate material in a particle bed from which particles are periodically added and withdrawn.
A wide variety of processes use radial or horizontal flow reactors to effect the contact of a compact bed of particulate matter with a fluid and in particular a gaseous stream. These processes include hydrocarbon conversion, adsorption, and exhaust gas treatment. In most of these processes, contact of the particulate material with the fluid decreases the effectiveness of the particulate material in accomplishing its attendant function. In order to maintain the effectiveness of the process, systems have been developed whereby particulate material is semi-continuously withdrawn from the contacting zone and replaced by fresh particulate material so that the horizontal flow of fluid material will constantly contact a compact bed of particulate material having a required degree of effectiveness. Typical examples and arrangements for such systems can be found in U.S. Pat. Nos. 3,647,680, 3,692,496, and 3,706,536, the contents of which are hereby incorporated by reference. A good example of the way in which moving bed apparatus has been used for the contacting of fluids and solids is found in the field of petroleum and petrochemical processes especially in the field of hydrocarbon conversion reactions. Many hydrocarbon conversion processes can also be effected with a system for continuously moving catalyst particles as a compact column under gravity flow through one or more reactors having a horizontal flow of reactants. One such process is the dehydrogenation of paraffins as shown in U.S. Pat. No. 3,978,150.
Another well known hydrocarbon conversion process that uses a radial flow bed for the contact of solid catalyst particles with a vapor phase reactant stream is found in the reforming of naphtha boiling hydrocarbons. This process uses one or more reaction zones wherein the catalyst particles enter the top of the reactor, flow downwardly as a compact column under gravity flow and are transported out of the first reactor. In many cases, a second reactor is located either underneath or next to the first reactor. Catalyst particles again move through the second reactor as a compact column under gravity flow. After passing through the second reactor, the catalyst particles may pass through additional reaction zones before collection and transportation to a regeneration vessel for the restoration of the catalyst particles by the removal of coke and other hydrocarbon by-products that accumulate on the catalyst in the reaction zone.
In the reforming of hydrocarbons using the moving bed system, the reactants typically flow serially through the reaction zones. The reforming reaction is typically endothermic so the reactant stream is heated before each reaction zone to supply the necessary heat for the reaction. The reactants flow through each reaction zone in a substantially horizontal direction through the bed of catalyst. The catalyst particles in each reaction zone are typically retained between an inlet screen and an outlet screen that together form a vertical bed and allow the passage of vapor through the bed. In most cases the catalyst bed is arranged in an annular form so that the reactants flow radially through the catalyst bed.
Experience has shown that the horizontal flow of reactants through the bed of catalyst can interfere with the gravity flow removal of catalyst particles. This phenomenon is usually referred to as hang-up or pinning and it imposes a constraint on the design and operations reactors with a horizontal flow of reactants. Catalyst pinning occurs when the frictional forces between catalyst pills that resist the downward movement of the catalyst pills are greater than the gravitational forces acting to pull the catalyst pills downward. The frictional forces occur when the horizontally flow vapor passes through the catalyst bed. When pinning occurs, it traps catalyst particles against the outlet screen of the reactor bed and prevents the downward movement of the pinned catalyst particles. In a simple straight reactor bed, or an annular bed with an inward radial flow of vapors, pinning progresses from the face of the outlet screen and as the vapor flow through the reactor bed increases, it proceeds out to the outer surface of the bed at which point the bed is described as being 100% pinned. Once pinning has progressed to the outermost portion of the catalyst bed, a second phenomenon called void blowing begins. Void blowing describes the movement of the catalyst bed away from an inlet screen by the forces from the horizontal flow of vapor and the creation of a void between the inlet screen and an outer catalyst boundary. The existence of this void can allow catalyst particles to blow around or churn and create catalyst fines. Void blowing can also occur in an annular catalyst bed when vapor flows radially outward through the bed. With radially outward flow, void blowing occurs when the frictional forces between the catalyst pills are greater than the gravitational forces, or in other words, at about the same time as pinning would occur with a radially inward flow. Therefore, high vapor flow can cause void blowing in any type of radial or horizontal flow bed.
The production of fines can pose a number of problems in a continuous moving bed design. The presence of catalyst fines increases the pressure drop across the catalyst bed thereby further contributing to the pinning and void blowing problems, can lead to plugging in fine screen surfaces, contributes to greater erosion of the process equipment, and in the case of expense catalysts imposes a direct catalyst cost on the operation of the system. Further discussion of catalyst fines and the problems imposed thereby can be found in U.S. Pat. No. 3,825,116 which also describes an apparatus and method for fines removal.
Where possible, horizontal or radial flow reactors are designed and operated to avoid process conditions that will lead to pinning and void blowing. This is true in the case of moving bed and non-moving bed designs. Apparatus and methods of operation for avoiding or overcoming pinning and void blowing problems are shown U.S. Pat. Nos. 4,135,886, 4,141,690, and 4,250,018.
Another problem that can effect radial flow reactors is fluidization of the upper particle bed surface and subsequent displacement or attrition of the catalyst particles. Fixed bed radial flow reactors commonly employ a variety of hold down methods to prevent fluidization of the top surface such as cover plates, inert packing material, or both. Typical cover plate and sealing arrangements for the top of fixed bed radial flow reactors are shown in U.S. Pat. Nos. 4,452,761 and 3,027,244. It has also been taught in a fixed bed arrangement to redirect a portion of the entering fluid from radial flow through the side of the bed to axial flow onto the top of the bed. This axial redirection of entering fluid provides containment of the upper bed surface as shown in U.S. Pat. No. 4,372,920. Moving bed reactors pose more difficulties since catalyst must be replaced while the upper surface of the bed remains in a sealed condition. High gas flows can be particularly disruptive and lead to fluidization and unwanted displacement of catalyst particles into other portions of the reactor internals. Complicated cover plate assemblies can stabilize the upper surface of the compact particle bed. U.S. Pat. No. 4,277,444 shows a system for confining an upper surface of a compact bed in a moving catalyst bed system. U.S. Pat. No. 5,130,106 shows a confining cover plate assembly that maintains downward pressure on the catalyst and prevents upward expansion of the bed.
Although the known cover plate assemblies can confine the upper bed surface and limit or prevent fluidization and churning of the catalyst, the transfer of catalyst in confined assemblies has resulted in occasional problems of cracking at weld seems due to thermal fatigue. The failures usually occur at a weld joint between a center screen and an imperforate screen section (hereinafter referred to as a xe2x80x9cblank-offxe2x80x9d) designed to retain a sealing layer of catalyst above the compact catalyst bed.
It is an object of this invention to simplify reactor internals for moving a compact bed of catalyst through a contacting vessel.
It is a further object of this invention to provide a system that prevents cracking problems at joints between screen and blank off elements in reactor internals.
Recent discoveries show that the cycling of the catalyst into the bed of the contacting zone results in thermal fatigue that leads to the above mentioned cracking problems. Cold catalyst that enters the sealed zone from above cools the hot gas in the seal zone along with the adjacent screen elements. As the hot seal gas heats up the catalyst, the temperature of the containment elements reaches the temperature of the hot gas until another transfer of catalyst occurs and the cycle repeats itself.
This invention is a change in the seal designs that improves the life of the containment elements and simplifies the overall design of the contacting vessel internals at the top of the compact bed. The invention modifies the seal at the top of the compact bed to heat catalyst before it contacts the screen section. The invention maintains the upper surface of the catalyst bed open to an entering flow of seal gas. The seal gas is free to flow upwardly out of an annular distribution space that distributes the radial flow of fluid along the surface of the radial flow catalyst bed. The entire inlet side of the screen may be kept completely open to allow full flow of the process fluid therethrough. The modification to the seal design eliminates most if not all of the blank-off from the outer screen. Eliminating the blank from the screen routes at least a portion of the seal gas horizontally across bed in addition to its usual vertical flow direction. This rerouting of the seal gas flow brings in more seal gas and preheats the cold catalyst, thereby reducing the temperature extremes of any thermal cycle. The addition of a small blank-off band at the top of the catalyst bed can protect the uppermost bed surface from any disturbance caused by the inflow of process gas which could create surface instability at the point where the entering radial gas flow at the upper surface of the bed is about equal to any downward axial flow of gas for containment of the bed.
The invention may also improve the reliability of the junction between screen elements by providing a solid piece of blank off that extends over any welded sections. A weld blank-off section may be beneficially added to cover any weld junction in the inlet screen. A narrow welded blank-off section covering any weld on the screen and separating the weld from the particulate material that passes through the bed and from the seal gas flow can provide enough temperature moderation to significantly reduce stresses in the weld area. Therefore, by extending the solid piece of blank-off for a short distance above and below the weld the maximum thermal stress moves away from the weld junction.
An intermediate blank-off in the inlet screen can also provide multiple advantages. The intermediate blank-off occupies a space at the bottom of the seal zone and separates the lower section of the perforated inlet screen from the upper section of the perforated screen located in the seal zone. Such a blank-off provides the advantage of forcing seal gas higher into the relatively colder catalyst located in the upper portion of the seal zone. Forcing seal gas higher into the seal volume produces a flow field that is closer to cocurrent flow rather than the typical cross-flow. A closer approach to cocurrent flow provides a more effective heat transfer, reduces the thermal cycle, and increases the axial flow relative to the radial flow at the inner screen outlet. Incorporation of the intermediate blank-off can also result in a higher local pinning margin, i.e. more tolerance against pinning, where seal gas enters the center pipe. Specifically, the intermediate blank-off provides for more efficient utilization of the seal gas thereby tending to reduce pinning at the inner screen.
In a broad embodiment, this invention is a process for passing a fluid in a cross flow direction through a compact bed of particulate material and periodically replacing the catalyst particles while maintaining the flow of the fluid. The process confines a particulate material in a vertically extended bed between an inlet partition having perforations extending over at least a part of its height and an outlet partition having a lower perforated section and upper section. Sizing of the perforations retains the particulate material while permitting gas flow. A first portion of the fluid stream passes into the bed in a direction substantially transverse to the direction of catalyst movement. The first portion passes through a first perforated section of the inlet partition located at or below the imperforate section of the outlet partition. A second fluid portion of the fluid stream passes through a second perforated section of the inlet partition located above the perforated portion of the outlet partition. The process recovers a third portion of the fluid stream from the perforated section of the outlet partition. At least periodically the process withdraws particulate material from the bottom of the vertically extended bed and adds particulate material to the top of the vertically extended bed. Optionally, a fourth portion of the fluid stream passes in substantially unrestricted flow to the top of the particles in the vertically extended bed.
In another embodiment this invention is an apparatus for passing a fluid in a cross flow direction through a compact bed of particulate material and periodically replacing the catalyst particles while maintaining the flow of the fluid. The apparatus comprises a vertically elongated inlet partition and a vertically elongated outlet partition, in a spaced apart relationship, that at least partially define a particle retention space and a retention space opening at an upper locus of the partitions for passing particles into the retention space. At least part of the inlet partition defines perforations. The outlet partition defines an upper imperforate section and a lower perforated section. The size of the perforations in both partitions retains particles while permitting fluid flow. A first perforated section of the inlet partition located at or below the imperforate section of the outlet partition at least partially defines a distribution chamber. A substantially open flow channel between the retention space opening and the distribution chamber provides substantially unrestricted flow to the retention space opening from the distribution chamber. A second perforated section of the inlet partition located above the perforated portion of the outlet partition has substantially unrestricted communication with the distribution chamber and the retention space opening. The perforated portion of the outlet partition defines at least part of a collection chamber. The apparatus also provides means for passing particulate material into the inlet opening.
In a more limited embodiment, this invention is a radial flow bed arrangement for contacting particulate material with a gaseous material. The arrangement comprises a vertically oriented center conduit having a plurality of perforations and a hollow interior for gas transfer; a vertically elongated retention screen surrounding the center conduit and together therewith defining an annular retention space between the center conduit and the retention screen for retaining particles; a vertically oriented center pipe welded to the top of the center conduit; an annular retention opening at the top of the annular retention space defined in part by the vertically oriented center pipe; a weld shroud comprising a narrow band encircling the outside of the center pipe and the center conduit at the weld; and, means for intermittently withdrawing particles from the bottom of the retention space and adding particles to the retention space.
Other objects, embodiments, and details of this invention are disclosed in the following detailed description.